1. Field of the Invention
This invention relates to an improvement in vacuum switch tubes used for switching a large electric current.
2. Description of the Prior Art
One example of a conventional vacuum switch tube is disclosed in Japanese Patent Application Laid-Open Specification No. 2-142024 (1990) corresponding to U.S. Pat. No. 5,059,752, and is constructed as shown in FIGS. 1 and 2. In the figures, reference numeral 1 denotes an insulating vacuum vessel evacuated to a high vacuum pressure of 10.sup.-4 Torr or below. A stationary electrode rod 4 is projecting downwardly from an upper stationary-side end plate 2 of the vacuum vessel 1. A movable electrode rod 5 is vertically movably disposed in the vessel 1 in opposed relation to the electrode rode 4 and is passed through a lower movable-side end plate 3 of the vessel 1. A bellows 6 is fitted over the movable electrode rod 5 and is secured at one end thereof to the electrode rod 5 and at the other end to the movable-side end plate 3.
The stationary electrode rod 4 is provided at its distal end with a stationary electrode 7, which includes a main electrode 7a located at a central portion thereof, an auxiliary electrode 7b located at the periphery of the central portion and connected to the electrode rod 4, and spiral grooves 7c formed on the auxiliary electrode 7b. The main electrode 7a serves as a contact and current-passing portion when the vacuum switch tube is operated. Therefore, an electrode material containing a low melting point metal such as Bi or the like is used for the main electrode 7a so that the electrode 7a requires a small tripping force against welding thereof. On the other hand, a material capable of breaking a large current and having good withstand voltage performance is used for the auxiliary electrode 7b.
The movable electrode rod 5 is provided at its distal end with a movable electrode 8, which is brought into contact with the stationary electrode 7 when the movable electrode rod 5 is moved upwardly. The movable electrode 8 comprises a main electrode 8a located at a central portion thereof, an auxiliary electrode 8b located at the periphery of the central portion and connected to the electrode rod 5, and spiral grooves 8c formed on the auxiliary electrode 8b. The main electrode 8a serves as a contact and current-passing portion when the vacuum switch is operated. Therefore, an electrode material containing a low melting point metal such as Bi is used for the main electrode 8a so that the electrode 8a requires a small tripping force against welding thereof. On the other hand, a material capable of breaking a large current and having good withstand voltage performance is used for the auxiliary electrode 8b.
Shield 9 is adapted to absorb a metal vapor emitted from the electrodes 7, 8 and is disposed on each side inside the vessel 1.
The operation of the vacuum switch tube will now be described. When the magnitude of the current flowing is of the order of a load or overload current, separation of the movable electrode 8 from the stationary electrode 7 completes the cut-off of the current in the regions of the main electrodes 7a, 8a.
When the current, however, is of a large magnitude, such as in a shortcircuit situation, separation of the stationary and movable electrodes 7 and 8 from each other causes an arc (not shown) to be generated between the main electrodes 7a and 8a. The arc generated becomes concentrated and is moved outwardly under the influence of a magnetic field developed by an external wiring or the like. Upon reaching the auxiliary electrodes 7b, 8b, the arc is given a rotating force by the spiral grooves 7c, 8c and is rotated around the center axis of the electrodes 7, 8 while moving further outwardly. This rotational motion of the arc prevents the same from stagnating locally to fuse and damage the electrodes 7, 8.
The prior art vacuum switch tube constructed as mentioned above shows insufficient withstand voltage performance, the main electrodes 7a, 8a of the stationary and movable electrodes 7, 8 containing a low melting point metal such as Bi or the like. Further, in the prior art vacuum switch tube, the generation of an arc between the main electrodes 7a and 8a is accompanied by a continuous emittance of a metal vapor from the low melting point metal in the main electrodes 7a, 8a, making it difficult for the arc to become concentrated. As a result, the arc tends to stagnate and fuse locally. Under such circumstances, a vacuum switch tube large in size has conventionally been used when breaking a large current.